With development of optical device integration technologies, a size of an optical device can reach a micron scale currently, so that a cross-sectional area of a waveguide, in the optical device, for transmitting an optical signal also reaches a micron scale. In this case, two components of the optical signal, that is, a transverse electric (TE) mode and a transverse magnetic (TM) mode, have great differences in terms of an effective refractive index difference, a group refractive index, and the like, resulting in generation of polarization dependent loss (PDL) and polarization mode dispersion (PMD) when the optical signal is transmitted in the optical device. In this case, the optical device is referred to as a polarization dependent optical device. When an optical signal is transmitted in the polarization dependent optical device, quality of the optical signal deteriorates due to existence of the PDL and the PMD. Therefore, a polarization independent characteristic is an essential requirement for an optical device in fiber optic communications. In addition, in a wavelength division system of the fiber optic communications, add/drop multiplexing usually needs to be performed on signals at different wavelengths. In other words, optical signals at different wavelengths need to be separated and combined. Therefore, how to construct a polarization independent optical device for performing optical signal add/drop multiplexing has drawn widespread attention.
FIG. 1 is a schematic structural diagram of an optical device for implementing optical signal add/drop multiplexing in a related technology. The optical device may be configured to drop optical signals at any wavelength in a wavelength division multiplexing (WDM) signal, or configured to add optical signals at different wavelengths. As shown in FIG. 1, when the optical device is configured to separate a WDM signal, the optical device includes one input port (Input), n output ports Rλ1, Rλ2, . . . , and Rλn, and n microring filter units 1001, 1002, . . . , and 100n. The microring filter unit 1001 is configured to separate an optical signal at a wavelength λ1, the microring filter unit 1002 is configured to separate an optical signal at a wavelength λ2, . . . , and the microring filter unit 100n is configured to separate an optical signal at a wavelength kn. The microring filter unit 1001 is used as an example to illustrate a process of separating a WDM signal by the optical device, where the WDM signal includes n optical signals at different wavelengths λ1, λ2, . . . , and λn. When the WDM signal is input from the input port, an optical signal at each wavelength in the WDM signal is split into two components, that is, a TE mode and a TM mode, by using a polarization beam splitter (PBS) 101. The TE mode of each optical signal is marked as QTE, and the TM mode is marked as PTM. PTM is converted to a TE polarization mode by using a polarization rotator (PR) 102, and the TE polarization mode is marked as PTE. In addition, a microring 103 included in the microring filter unit 1001 includes four ports: an input port, a throughput port, an add port, and a drop port. A QTE is input from the input port of the microring 103. When a wavelength of an optical signal corresponding to the QTE is λ1 that meets a resonance condition of the microring 103, the QTE is output from the drop port of the microring 103. A PTE corresponding to the QTE is input from the throughput port of the microring 103, is output from the add port of the microring 103, and is converted to TM polarization PTM again by using another PR 104. The QTE and the PTM are combined, and an optical signal at the wavelength λ1 is output from the output port Rλ1, so as to complete separation of the optical signal at the wavelength λ1, that is, to drop the optical signal at the wavelength λ1. The n optical signals at different wavelengths included in the WDM signal are separately separated by the n microring filter units 1001, 1002, . . . , and 100n.
Similarly, when the optical device is configured to combine optical signals at different wavelengths, the foregoing n output ports Rλ1, Rλ2, . . . , and Rλn may act as n input ports, of the optical device, for lightwaves at different wavelengths, and the foregoing input port (Input) may act as an output port for a combined optical signal. Then, the n optical signals at different wavelengths are combined by using the n microring filter units 1001, 1002, . . . , and 100n, that is, the n optical signals at different wavelengths are added by using the optical device. Polarization beam splitting and polarization rotation are performed on an optical signal before the optical signal enters a microring filter unit, so that the optical signal passing through the microring filter unit includes only one polarization mode. Therefore, even if a device included in the optical device is a polarization dependent device, no PDL or PMD is generated when the optical signal is transmitted in the optical device. In other words, the optical device is a polarization independent optical device.
When the optical device shown in FIG. 1 is used to separate a WDM signal, the optical device cannot be used to combine optical signals at different wavelengths; when the optical device is used to combine optical signals at different wavelengths, the optical device cannot be used to separate a WDM signal. Therefore, the optical device shown in FIG. 1 severely limits an optical signal add/drop multiplexing function.